Low-alloy heat-resistant steel having high reheat-cracking resistance

ABSTRACT

Steel, including the following element ratios (in mass percent amounts): carbon (C): 0.04-0.11%; silicon (Si): 0.50% or less; manganese (Mn): 0.10-0.60%; phosphorus (P): 0.03% or less; sulphur (S): 0.01% or less; nicide (Ni): 0.40% or less; chromium (Cr): 1.90-2.60%; vanadium (V): 0.20-0.30%; niobium (Nb): 0.02-0.08%; molybdenum (Mo): 0.05-0.30%; tungsten (W): 1.45-1.75%; titanium (Ti): 0.01-0.06%; boron (B): 0.001-0.012%; aluminum (Al): 0.03% or less; nitrogen (N): 0.01% or less; the balance being iron (Fe) and impurities. The contents of C and B satisfy the following inequality: (% B) −1.2×(% C)2+0.30×(% C)−0.01.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119 and the Paris Convention Treaty, this application claims foreign priority to Chinese Patent Application No. 201811462079.4 filed Nov. 30, 2018, the contents of which, including any intervening amendments thereto, are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P.C., Attn.: Dr. Matthias Scholl Esq., 245 First Street, 18th Floor, Cambridge, Mass. 02142.

BACKGROUND

The disclosure relates to a heat-resistant steel, which has excellent resistance to reheat cracks, and is insensitive to intergranular cracks.

The reheat cracking, also referred to as a post-weld heat treatment cracking or a stress relief cracking, refers to intergranular cracking generated in a coarse-grained heat affected zone (CGHAZ) of a welded joint of a metal material due to stress release during a post-weld heat treatment or high-temperature service.

Conventional low-alloy (i.e., having less than 5 wt. % of alto-resistant steels, such as T23, T24, Cr—Mo, and Cr—Mo—V, tend to reheat cracking.

SUMMARY

The disclosure provides steel, which has high reheat cracking resistance in a coarse-grained heat affected zone while maintaining excellent high-temperature creep strength, and is insensitive to reheat cracking during a post-weld heat treatment or service at temperatures of 500 to 750° C.

Disclosed is steel, comprising the following element ratios, stated in mass percent amounts: carbon (C): 0.04-0.11%; silicon (Si): 0.50% or less; manganese (Mn): 0.10-0.60%; phosphorus (P): 0.03% or less; sulphur (S): 0.01% or less; nickle (Ni): 0.40% or less; chromium (Cr): 1.90-2.60%; vanadium (V): 0.20-0.30%; niobium (Nb): 0.02-0.08%; molybdenum (Mo): 0.05-0.30%; tungsten (W): 1.45-1.75%; titanium (Ti): 0.01-0.06%; boron (B): 0.001-0.012%; aluminum (Al): 0.03% or less; and nitrogen (N): 0.01% or less, the balance being iron (Fe) and impurities; where the contents of carbon (C) and boron (B) satisfy the following inequality:

(% B)>−1.2×(% C)²+0.30×(% C)−0.01   (1).

Unless otherwise stated, the percent (%) of chemical compositions in the disclosure refers to a mass percent.

The content of C in the steel is 0.04-0.08%, and the content of B in the steel can be 0.004-0.01%.

The content of C in the steel can be 0.04-0.08%, and the content of B in the steel can be 0.004-0.008%.

The content of B in the steel can be 0.004-0.012%.

The content of B in the steel can be 0.006-0.010%.

The contents of C and B in the steel satisfy the following inequality:

(% B)>−1.4×(% C)²+0.35×(% C)−0.0115   (2).

Also provided is a heat-resistant steel comprising the following element ratios in mass percent amounts: C: 0.04-0.08%, Si: 0.50% or less, Mn: 0.10-0.60%, P: 0.03% or less, 5: 0.01% or less, Ni: 0.40% or less, Cr: 1.90-2.60%, V: 0.20-0.30%, Nb: 0.02-0.08%, Mo: 0.05-0.30%, W: 1.45-1.75%, Ti: 0.01-0.06%, B: 0.001-0.012%, Al: 0.03% or less, and N: 0.01% or less, the balance being Fe and impurities, where the contents of C and 13 satisfy the following inequality:

(% B)>−1.4×(% C)²+0.35×(% C)−0.0115   (2)

The content of B in the steel can be 0.004-0.01%.

The content of B in the steel can be 0.006-0.01%.

The function of each element in the steel of the disclosure and the effects of the range of each element will be described below

C: 0.04-0.11%

C is formed into carbide in the steel, which is good for high-temperature strength. Further, C helps to improve hardenability and avoid forming a ferrite. Therefore, the content of C is at least 0.04%. However, the excessive C increases the hardness of a heat affected zone of welded joint, and increases sensitivity to cold cracks, and particularly contributes to reheat cracking. In addition, steel with a high content of C may become brittle after being used for a long time at high temperature. Therefore, an upper limit of the content of C is 0.11%, and preferably 0.04-0.08%.

Si: 0.50% or less

Si serves as a deoxidizing element in steel manufacture. Si is also effective for improving oxidation resistance and high-temperature corrosion resistance of the steel. However, the excessive content of Si may result in reduction of creep plasticity and toughness during long-term use at high temperature. Therefore, the upper limit of the content of Si is 0.50%, and a lower limit is an unavoidable impurity content level. To ensure deoxidization effect, the content of Si preferably is 0.10-0.30%.

Mn: 0.10-0.60%

Like Si, Mn is added as a deoxidizing agent. However, excessive addition of Mn may result in creep embrittlement and the reduction of toughness. Therefore, the maximum content of Mn is 0.60%. To ensure the deoxidization effect, the content of Mn preferably is 0.20-0.50%.

P: 0.03% or less

P exists as an unavoidable impurity in the steel. The high content of P may easily result in reheat cracking. Therefore, the maximum content of P is 0.03%. The content of P preferably is as low as possible, and therefore, no lower limit is set for the content. However, excessive reduction of the content of P may increase manufacturing cost, and thus the content of P preferably is 0.001-0.01%.

S: 0.01% or less

Like P, S also exists as an unavoidable impurity. S may be easily segregated in the CGHAZ, thereby resulting in the generation of reheat cracking. Therefore, the content of S is limited to 0.01% or less. The content of S preferably is as low as possible, and therefore no lower limit is set for the content. However, like P, excessive reduction of the content of S may increase the manufacturing cost, and thus the content preferably is 0.002-0.006%.

Ni: 0.4%© or less

Ni is an austenite forming element. Ni inhibits the formation of a 8 ‘ferrite phase and ensures the stability of a ferrite structure. Excessive addition of Ni may reduce the plasticity during high-temperature use. Therefore, the content of Ni is limited to 0.4% or less.

Cr: 1.90-2.60%

Cr is an indispensable element for ensuring high-temperature oxidization resistance, high-temperature corrosion resistance and high-temperature strength. However, excessive addition of Cr may coarsen the carbide, thereby finally resulting in the reduction of high-temperature strength and toughness. Therefore, the content of Cr is limited to 1.90-2.60%.

‘V: 0.20-0.30%

V is formed into fine carbides or carbonitrides in the steel, and helps to increase creep strength. However, excessive addition of V may result in increased growth speed of the carbides, premature aggregation and coarsening, and as a result premature disappearance of dispersion strengthening and toughness reduction. In addition, excessive addition of V may increase precipitation density of the carbonitrides inside grain during a post-weld heat treatment and increase the reheat cracking sensitivity. Therefore, the content of V is limited to 0.20-0.30%.

Nb: 0.02-0.08%

Nb is formed into fine and stable carbides or carbonitrides in the steel, and helps to increase creep strength. Therefore, it is necessary to add at least 0.02% of Nb. However, excessive addition of Nb may result in the increased growth speed of the carbides, premature aggregation and coarsening, and consequently premature disappearance of dispersion strengthening and toughness reduction. Therefore, the content of Nb is limited to 0.02-0.08%.

Mo: 0.05-0.35%

Mo increases the solid solution strength of steel matrix, and is precipitated in the form of carbides to increase the creep strength. Further, Mo has strong affinity for P, and may reduce an amount of P segregated at a grain boundary, thereby helping to reduce the reheat cracking sensitivity. Therefore, it is necessary to control the content of Mo to 0.05% or more. However, excessive addition of Mo may reduce the toughness of the steel after long-term exposure and thus, the upper limit of Mo is 0.35%.

W: 1.45-1.75%

Like Mo, W increases the solid solution strength of the steel matrix, and is formed into carbides to increase the creep strength. To obtain these effects, the content of W is at least 1.45%, but excessive addition may result in the generation of coarse intermetallic compounds during service and further reduce toughness. Therefore, the content is limited to less than 1.75%.

Ti: 0.01-0.06%

The addition of Ti may bind N to prevent a combination of N and B. In this way, the hardena.bility is improved to prevent the reduction of tensile strength due to the generation of the ferrite, and also grain boundary strength and plasticity at a high temperature are increased. However, the excessive content of Ti may reduce the strength and toughness, and also increase the reheat cracking sensitivity. Therefore, the content of Ti is in a range of 0.01-0.06%.

B:0:001-0.012%

B can increase the creep strength and the creep fracture plasticity. Further, during the post-weld heat treatment, B can increase the grain boundary plasticity through grain boundary segregation in the CGHAZ and also can inhibit the precipitation, growth and coarsening of the carbides, prevent the grain boundary from weakening, and reduce the reheat cracking sensitivity. To be effective, the contents of B and C must satisfy (% B)−1.2×(% C)²+0.30×(% C)−0.01. Preferably, when the contents of B and C satisfy (% B) >−1.4×(% C)²+0.35×(% C)−0.0115, better reheat cracking resistance can be achieved. The excessive content of B may obviously deteriorate hot workability of the steel and also cause temper embrittlement. Therefore, the content of the element B appropriately is 0.001-0.012%, preferably is 0.004-0.010%, and more preferably is 0.006-0.010%.

Al: 0.03% or less

Al is included as a deoxidizing agent. However, since the excessive content may result in the reduction of creep plasticity and toughness, the content of Al is limited to 0.03% or less.

N: 0.01% or less

N is soluble in the matrix and thus is harmful to the toughness and creep strength. Further, the excessive content of N may form compounds with B, which is not favorable to exerting the function of B. Therefore, the content of N is limited to 0.01% or less.

Advantages of the steel of the disclosure are summarized as follows. The purpose of inhibiting reheat cracking of a low-alloy heat-resistant steel can be achieved by adjusting the contents of two elements C and B, only. Compared with the demanding welding process adopted for preventing reheat cracking, the disclosure solves the problem of reheat cracking sensitivity, and does not increase production cost in order to achieve a more reliable result.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relationship of M₂₃C₆, an MX type carbide, and a carbon content in a 2.25Cr-1.6WVNbNB steel.

FIG. 2 illustrates a microscopic structure of steel.

FIG. 3 illustrates a sample shape in an implant test.

FIG. 4 illustrates a simulated thermal cycle curve and a sample shape and size.

FIG. 5 is a schematic diagram illustrating a relationship between the contents of B and C of a heat-resistant steel according to one embodiment of the disclosure.

In the drawings, the following reference numbers are used: 1. Bottom plate; 2. Implant; and 3. Loading direction.

DETAILED DESCRIPTION

To further illustrate, embodiments detailing a heat-resistant steel are described below. It should be noted that the following embodiments are intended to describe and not to limit the disclosure.

The disclosure provides a low-alloy heat-resistant steel with reheat cracking resistance, which greatly improves a reheat cracking resistance while maintaining excellent high-temperature creep strength and low-welding cold cracking sensitivity of the new low-alloy heat-resistant steel. The steel can be safely and reliably used in an ultra (super) critical thermal power plant.

Chemical composition of steel influences its sensitivity to reheat cracking. In particular, alloy elements such as C and W, Mo, V, Nb and Ti may all increase the tendency to undergo reheat cracking. However, W and Mo are soluble in the steel matrix and provide solid solution strengthening, the V, Nb, and Ti are formed into fine dispersed carbonitrides to provide precipitation strengthening, and thus they are key elements for increasing a high-temperature creep strength of a material. To obtain sufficient high-temperature creep strength, it is necessary to add a certain amount of these alloy elements, The inventor of the disclosure studied the reheat cracking generation mechanism of the low-alloy heat-resistant steel T23 and found that the content of impurity elements in this steel can be controlled to be in a low range with the development of steel manufacturing technology and that this is no longer a leading factor for generating reheat cracking, but that the precipitation of alloy carbides during a post-weld heat treatment has a significant impact on the generation of reheat cracking. The above is different from the past viewpoint that emphasizes the segregation of impurities elements on the grain boundary of the conventional low-alloy heat-resistant steel, and weakens the grain boundary, resulting in the generation of reheat cracks. Thus, a new basis for developing a new low-alloy heat-resistant steel with reheat cracking resistance is provided.

The following findings are offered with respect to the reheat cracking mechanism of the CGHAZ in T23 steel.

(1) Most of the carbides in the CGHAZ are soluble in the matrix during welding, and M₂₃C₆ type carbides containing elements such as Fe, Cr, W and Mo are precipitated along the grain boundary, rapidly grown, and coarsened to reduce the binding force between grain boundaries during a post-weld heat treatment.

(2) A relatively large amount of coarse M₂₃C₆ type carbides precipitated on the grain boundary within short time may result in a depletion of alloy elements in the nearby matrix, thereby forming a softening zone in which strain is preferentially aggregated under tensile stress at high temperature.

(3) The incoherent M₂₃C₆ carbides may also promote nucleation of a creep cavity on the grain boundary to further accelerate the weakening of the grain boundary.

(4) During a short-time post-weld heat treatment, carbides precipitated in the grain interior mainly are M₂₃C₆ and M₃C₃ having small sizes, and may have a certain strengthening effect, thereby increasing intragranular strength.

(5) The intra-granular strength is high and the grain boundary is obviously weakened. The grain boundary of the CGHAZ preferentially deforms to form a cavity under the action of welding residual stress as well as thermal stress, and the cavities are aggregated to form micro-cracks and propagate further to cause inter-granular fracture.

Carbides formed by elements such as Cr, W, and Mo have a large impact on reheat cracking of the heat-resistant steels, but the elements such as V, Nb and Ti have a small impact on reheat cracking. Therefore, from the perspective of inhibiting the reheat cracking, the precipitation of M₂₃C₆ type carbides on the grain boundary and the grain interior of the CGHAZ are to be mainly controlled.

Carbon is an essential element for forming M₂₃C₆, the higher carbon content of the steel may cause more carbon atoms to form carbides in the CGHAZ during a post-weld heat treatment. Therefore, a precipitation amount of CGHAZ carbides may be limited by limiting the content of carbon. FIG. 1 illustrates a change of the contents of M₂₃C₆ and MX carbides in a steel T23 along with the content of the carbon element, where the contents of M₂₃C₆ and MX carbides and carbon are calculated by using Thermo-calc software (where a total amount of an alloy system is 1 mol, and a result of a metastable state is obtained by inhibiting the generation of an M₆C phase during calculation). As shown, the contents of both MX and M₂₃C₆ phases linearly increase along with the increase of the content of C, but the increase of the content of the M₂₃C₆ phase is greater than that of the MX phase. When the content of C increases by each 0.02%, the M₂₃C₆ type carbide in 1 mol of the steel increases by about 0.003 mol, and the MX phase increases only by about 0.0002 mol which is less than a tenth of the former. That is, reducing the content of C can greatly reduce the content of M₂₃C₆ type carbide, but has a small impact on the MX type carbide. The MX phase is a main precipitation strengthening phase in the grain and is extremely important for maintaining high-temperature creep strength. The strengthening effect of the MX phase in the grain interior may still be maintained since the appropriate control of the carbon content has a small impact on the MX phase, thereby ensuring the high-temperature creep strength. Therefore, it is appropriate to reasonably limit the carbon content in consideration of reducing the content of carbides on the grain boundary and lessening the weakening of the grain boundary of the CGHAZ during the post-weld heat treatment.

Further, the weakening of the grain boundary caused by grain boundary carbides is related to the size and distribution of the carbides. The coarse carbides may reduce coherence and cause severe depletion of alloy elements in the matrix near the grain boundary, thereby intensifying the weakening of the grain boundary. However, the reduction of coherence and the depletion of elements caused by the fine carbides are much smaller. The reheat cracking sensitivity of the steel T23 is high because the M₂₃C₆ type carbides precipitated on the grain boundary of the CGHAZ during a post-weld heat treatment are extremely easily aggregated and coarsened. The element B is easily segregated in a vacancy of the grain boundary in the steel to inhibit the segregation of impurity elements, reduce activation energy of the grain boundary, purify and strengthen the grain boundary, and improve the plasticity of the grain boundary. The element B can also enter into the M₂₃C₆ phase to form a more stable M₂₃(C, B)₆ phase to inhibit its coarsening. The addition amount of B is related to the content of C in the steel. The higher content of C indicates that more B needs to be added to inhibit the aggregation and coarsening of the M₂₃C₆ phase.

Therefore, the contents of C and. B in the steel are controlled. This reduces the amount and the size of the M₂₃C₆ type carbides precipitated in the grain boundary during a post-weld heat treatment, improves the plasticity of the grain boundary, and reduces the difference between grain boundary strength and intragranular strength, thereby inhibiting the generation of reheat cracking.

FIG. 5 obtained by using data in the following several examples illustrates a relationship between the content of B existing in the steel and the generation of reheat cracks relative to the content of C. In FIG. 5, the horizontal axis refers to the content of C in the steel according to examples of the disclosure or comparison examples, and the longitudinal axis refers to the content of B in the steel. A hollow circle (a) refers to steels with no reheat cracks, and a solid circle (●) refers to steels with reheat cracks. It is determined from FIG. 5 that the relationship between the content of C (% C) and the content of B (% B) satisfies the following inequality:

(% B)−1.2×(% C)²+0.30×(% C)−0.01   (1),

and thus, the reheat cracks can be prevented.

Steels comprising chemical compositions (containing a maximum total amount of 0.04% of impurities such as Sn, As, Sb, Bi and Pb) shown in Table 1 are prepared. Steel billets are obtained by electric arc furnace refining plus external refining and vacuum degassing or smelting by an electroslag remelting process. Bars with a size being 55 mm×55 mm×800 mm are manufactured by hot forging; pipes with different specifications are manufactured by perforation and hot rolling, outer diameters of the pipes cover a plurality of sizes from 38.1 mm to 63.5 mm, and wall thickness covers a plurality of sizes from 4.5 mm to 10 mm. Heat treatment of normalization and tempering are performed for plates and pipes. The normalizing temperature was 1060° C., and the plates and pipes were air-cooled after being incubated for 2 hours. The tempering temperature is 760° C., and the plates and pipes are cooled in the furnace after being incubated for 1 hour. A microstructure of the steel of the disclosure is a full bainite steel, as shown in FIG. 2.

The reheat cracking sensitivity was evaluated by performing a test with a sample. The reheat cracking sensitivity of the steel is evaluated by three methods in the disclosure, and the methods comprise a post-weld heat treatment test of welded joints performed in a waterwall pannel and a header, an implant test and an isothermal slow strain rate tensile test of simulated CGHAZ.

A process of performing an actual post-weld heat treatment test of a joint comprises: a welded joint of tube is released residual stress by performing heat treatment at 730±10° C. for 0.5 hours after welding, and performing magnetic powder inspection and X-ray inspection for the joint to detect whether cracks are generated on a surface of the joint or inside the joint.

An implant sampling process comprises: processing samples of an implant 2 and a bottom plate 1 as shown in FIG. 3, assembling the implant 2 into the central hole of the bottom plate 1, and applying a welding pass on the bottom plate. The implant and the bottom plate that are welded are placed for 24 hours to eliminate the impact of cold cracks, then installed on an implant tester, heated to a testing temperature and incubated for 15 minutes, and then loaded with a particular initial stress. The reheat cracking sensitivity may be determined by fracture time (t_(f)). If t_(f) is less than 24 hours, it is considered to be sensitive; if t_(f) is greater than 24 hours, it is considered to be insensitive. The shorter the fracture time is, the larger the reheat cracking sensitivity is.

A process of performing the isothermal slow strain rate tensile test comprises: processing samples as shown in FIG. 4, using the range of 10 mm in the middle as a gauge length portion, and completing a welding simulation test of a CGHAZ on a thermal simulator. A simulated thermal cycle curve of welding is shown in FIG. 3, and a simulated welding process is as follows: TIG welding process, preheating at 100° C., and a heat input being 25 kJ/cm, After being cooled, the sample is heated to a test temperature of 500 to 750° C., incubated for 5 seconds, and then strained at a constant strain rate of 0.5 mm/min until it is fractured, and a reduction of area of the fractured sample is measured. According to the reduction of area (Z) of the fractured sample, the reheat cracking sensitivity of the material may be generally determined based on the following criteria: 1) the material is very sensitive when Z is less than 5%; 2) the material is sensitive when Z is greater than 5% and less than 10%; 3) the material is slightly sensitive when Z is greater than 10% and less than 20%; 4) the material is insensitive when Z is greater than 20%. In the disclosure, it is considered as non-cracked when Z is greater than 20% at all test temperatures and otherwise it is considered as cracked.

TABLE 1 Chemical compositions of steels in examples (wt. %) Chemical compositions (mass percent; balance: Fe and impurities Steel C Si Mn P S Ni Cr V Nb Mo Comparison  1# 0.052 0.16 0.35 0.006 0.004 0.055 2.22 0.25 0.038 0.11 examples  2# 0.061 0.23 0.35 0.010 0.003 0.014 2.31 0.26 0.056 0.11  3# 0.068 0.23 0.33 0.008 0.003 0.023 2.19 9.24 0.041 0.11  4# 0.068 0.20 0.37 0.008 0.002 0.055 2.35 0.26 0.029 0.13  5# 0.072 0.25 0.36 0.014 0.001 0.022 2.28 0.25 0.050 0.12  6# 0.074 0.23 0.37 0.010 0.006 0.023 2.29 0.23 0.040 0.10  7# 0.049 0.25 0.44 0.014 0.001 0.020 2.25 0.24 0.050 0.11  8# 0.078 0.24 0.38 0.007 0.002 0.030 2.28 0.25 0.030 0.12  9# 0.083 0.16 0.40 0.007 0.001 0.040 2.04 0.23 0.045 0.13 10# 0.076 0.24 0.36 0.011 0.007 0.028 2.16 0.24 0.030 0.09 11# 0.085 0.17 0.35 0.004 0.003 0.005 2.16 0.25 0.038 0.11 12# 0.092 0.16 0.40 0.007 0.001 0.040 2.04 0.23 0.045 0.13 13# 0.10  0.14 0.46 0.014 0.001 0.040 2.20 0.25 0.050 0.12 Examples 14# 0.050 0.27 0.39 0.005 0.002 0.037 2.30 0.24 0.050 0.08 of the 15# 0.056 0.17 0.35 0.005 0.002 0.10  2.15 0.25 0.038 0.12 disclosure 16# 0.050 0.25 0.40 0.004 0.003 0.024 2.24 0.25 0.044 0.12 17# 0.060 0.18 0.35 0.004 0.003 0.002 2.19 0.25 0.040 0.11 18# 0.067 0.21 0.35 0.006 0.002 0.080 2.15 0.24 0.038 0.12 19# 0.075 0.18 0.36 0.005 0.003 0.002 2.18 0.25 0.038 0.12 20# 0.096 0.23 0.38 0.005 0.004 0.012 2.11 0.24 0.042 0.11 21# 0.054 0.18 0.36 0.005 0.002 0.060 2.18 0.25 0.042 0.12 22# 0.060 0.24 0.18 0.017 0.005 0.020 2.18 0.25 0.040 0.07 23# 0.041 0.16 0.35 0.004 0.003 0.003 2.16 0.24 0.035 0.11 24# 0.043 0.16 0.35 0.004 0.003 0.003 2.20 0.25 0.037 0.11 25# 0.050 0.21 0.38 0.006 0.003 0.050 2.26 0.26 0.055 0.09 26# 0.061 0.25 0.22 0.020 0.005 0.010 2.15 0.25 0.038 0.08 27# 0.070 0.18 0.33 0.005 0.001 0.003 2.17 0.25 0.036 0.10 28# 0.075 0.18 0.38 0.006 0.003 0.002 2.18 0.25 0.038 0.11 29# 0.064 0.16 0.42 0.004 0.001 0.004 2.13 0.24 0.042 0.11 30# 0.080 0.18 0.32 0.004 0.002 0.005 2.15 0.25 0.040 0.11 31# 0.089 0.17 0.35 0.004 0.002 0.003 2.18 0.25 0.038 0.12 32# 0.105 0.25 0.42 0.004 0.003 0.008 2.25 0.25 0.044 0.10 Chemical compositions (mass percent; balance: Fe and impurities) −1.2 * C² + Steel W Ti Al B N 0.3 * C − 0.01 Comparison  1# 1.50 0.002 0.020 0.0012 0.008 0.0024 examples  2# 1.56 0.004 0.018 0.0022 0.006 0.0038  3# 1.51 0.004 0.003 0.0011 0.006 0.0049  4# 1.63 0.002 0.020 0.0026 0.006 0.0049  5# 1.53 0.003 0.001 0.0013 0.004 0.0054  6# 1.56 0.005 0.013 0.0030 0.006 0.0056  7# 1.60 0.013 0.001 0.0010 0.003 0.0018  8# 1.60 0.014 0.030 0.0023 0.004 0.0061  9# 1.35 0.003 0.005 0.0036 0.001 0.0066 10# 1.50 0.004 0.006 0.0022 0.006 0.0059 11# 1.56 0.027 0.008 0.0065 0.003 0.0068 12# 1.35 0.003 0.005 0.0048 0.003 0.0074 13# 1.54 0.016 0.022 0.0070 0.008 0.0080 Examples 14# 1.56 0.038 0.009 0.0025 0.006 0.0020 of the 15# 1.56 0.029 0.013 0.0035 0.002 0.0030 disclosure 16# 1.55 0.033 0.007 0.0030 0.005 0.0020 17# 1.57 0.030 0.010 0.0045 0.001 0.0037 18# 1.56 0.025 0.013 0.0050 0.002 0.0047 19# 1.58 0.022 0.006 0.0063 0.002 0.0058 20# 1.61 0.025 0.008 0.0081 0.006 0.0077 21# 1.55 0.023 0.011 0.0060 0.004 0.0027 22# 1.50 0.025 0.003 0.0055 0.007 0.0037 23# 1.60 0.021 0.007 0.0010 0.002 0.0003 24# 1.57 0.024 0.004 0.0024 0.002 0.0007 25# 1.48 0.019 0.012 0.0060 0.005 0.0020 26# 1.49 0.022 0.004 0.0060 0.006 0.0038 27# 1.58 0.024 0.006 0.0072 0.006 0.0051 28# 1.56 0.023 0.006 0.0072 0.003 0.0058 29# 1.52 0.025 0.008 0.0070 0.007 0.0043 30# 1.58 0.032 0.006 0.0080 0.006 0.0063 31# 1.57 0.030 0.017 0.010  0.002 0.0072 32# 1.58 0.021 0.005 0.0092 0.006 0.0083

In the prepared steels, 1-13# are comparison examples, most of the compositions thereof are within a specified range of T23, and the contents of B and C do not satisfy a relationship of (% B)>(% B)>−1.2×(% C)²+0.30>(% C)−0.01. It can be seen from Table 2 that 1-34 are cracked in the post-weld heat treatment test of a pipe joint; 4-6# are sensitive to reheat cracking within a wide heat treatment temperature range based on the evaluation of the implant test; 7-9# are sensitive to reheat cracking within a wide heat treatment temperature range based on the evaluation of the implant test and the simulated CGHAZ isothermal slow strain rate tensile test ; 10# is cracked in the post-weld heat treatment test of a tube joint, and is sensitive to reheat cracking at 550 to 750° C. based on the evaluation of the simulated CGHAZ isothermal slow strain rate tensile test; 11-13# are all sensitive to reheat cracking within a particular heat treatment temperature range based on the evaluation of the simulated CGHAZ isothermal slow strain rate tensile test Based on the evaluation of the above three test methods, these steels have a very high reheat cracking tendency, reheat cracks are easily generated in the CGHAZ during the post-weld heat treatment or exposure at high temperature, and the reheat cracking resistance is very poor.

14-32# are steels designed according to the disclosure and have compositions satisfying the relationship of (% B)>−1.2×(%C)²+0.30×(% C)×0.01. 14-16# are not cracked in the actual joint post-weld heat treatment test, and no cracks are detected on a surface layer and inside the joint by non-destructive inspection as well. 17-18# are not cracked in the actual joint post-weld heat treatment test, and no cracks are detected on the surface layer and inside the pipe joint by the non-destructive inspection as well; 17-184 are insensitive to reheat cracking within a wide heat treatment temperature range (500 to 750° C.) based on the evaluation of the implant test. 19-20# are insensitive to reheat cracks within the temperature range of 500 to 750° C. based on the implant test. 21-23# are insensitive to reheat cracking within a temperature range of 500-750° C. based on the simulated CGHAZ isothermal slow strain rate tensile test. Therefore, it indicates that the steels with compositions satisfying the disclosure are all insensitive to reheat cracking, no reheat cracks are generated in the CGHAZ during the post-weld heat treatment or exposure at high temperature, and the reheat cracking resistance is excellent.

Test materials comprise bars and pipes with different specifications. The reheat cracking resistance is very good as long as the compositions of the steel satisfy the relationship of (% B)>−1.2×(% C)²+0.30×(% C)−0.01. Otherwise, the reheat cracking resistance of the test materials is very poor. Therefore, it indicates that the reheat cracking resistance of the test materials is not affected by a manner in which the materials are formed.

TABLE 2 Reheat cracking evaluation test results of steels in examples Determined as cracked Steel Test method Test result or not Comparison  1# Pipe joint post-weld heat Cracked in the post-weld heat Cracked examples  2# treatment test treatment process  3#  4# Implant test Sensitive at 600-750° C. Cracked  5# Sensitive at 650-750° C.  6# Sensitive at 650-750° C.  7# Implant test Sensitive at 700-750° C. Cracked Simulated coarse grain zone Sensitive at 650-770° C. short-time creep fracture test  8# Implant test Sensitive at 600-750° C. Cracked Simulated coarse grain zone Sensitive at 550-750° C. short-time creep fracture test  9# Implant test Sensitive at 600-750° C. Cracked Simulated coarse grain zone Sensitive at 550-750° C. short-time creep fracture test 10# Pipe joint post-weld heat Cracked in the post-weld heat Cracked treatment test treatment process Simulated coarse grain zone Sensitive at 550-750° C. short-time creep fracture test 11# Simulated coarse grain zone Sensitive at 700-750° C. Cracked 12# short-time creep fracture Sensitive at 550-750° C. 13# test Sensitive at 600-750° C. Examples 14# Pipe joint post-weld heat Not cracked in the post-weld Not cracked of the 15# treatment test heat treatment process disclosure 16# 17# Pipe joint post-weld heat Not cracked in the post-weld Not cracked 18# treatment test and implant heat treatment process and test both insensitive at 500-750° C. 19# Implant test Both insensitive at 500-750° C. Not cracked 20# 21# Simulated coarse grain zone All insensitive at 500-750° C. Not cracked 22# short-time creep fracture 23# test 24# 25# 26# 27# 28# 29# 30# 31# 32#

It will be obvious to those skilled in the art that changes and modifications may he made, and therefore, the aim in the appended claims is to cover all such changes and modifications. 

What is claimed is:
 1. Steel, comprising the following element ratios in mass percent amounts: carbon (C): 0.04-0.11%; silicon (Si): 0.50% or less; manganese (Mn): 0.10-0.60%; phosphorus (P): 0.03% or less; sulphur (5): 0.01% or less; nickle (Ni): 0.40% or less; chromium (Cr): 1.90-2.60%; vanadium (V): 0.20-0.30%; niobium (Nb): 0.02-0.08%; molybdenum (Mo): 0.05-0.30%; tungsten (W): 1.45-1.75%; titanium (Ti): 0.01-0.06%; boron (B): 0.001-0.012%; aluminum (Al): 0.03% or less; and nitrogen (N): 0.01% or less, the balance being iron (Fe) and impurities; wherein the contents of C and B, in mass percent amounts, satisfy the following inequality: (% B)>−1.2×(% C)²+0.30×(% C)−0.01.
 2. The steel of claim 1, content of C in the steel is 0.04-0.08%.
 3. The steel of claim 1, wherein a content of B in the steel is 0.004-0.01%.
 4. The steel of claim
 1. wherein a content ofB in the steel is 0.006-0.01%.
 5. The steel of claim
 1. wherein the contents of C and B satisfy the following inequality: (% B)>−1.4×(% C)²+0.35×(% C)−0.0115.
 6. The steel of claim 2, wherein the content of B in the steel is 0.004-0.01%.
 7. The steel of claim 2, wherein the content of B in the steel is 0.004-0.008%.
 8. The steel of claim 1, wherein a content of C in the steel is 0.04-0.08%, and the contents of C and B satisfy the following inequality: (% B)>−1.4(% C)²+0.35×(% C)−0.0115.
 9. The steel of claim 8, wherein the content of B in the steel is 0.004-0.01%.
 10. The steel of claim 8, wherein the content of B in the steel is 0.006-0.01%. 